The present invention relates generally to a digital impedance measurement system and, in particular, to a digital eddy current system used to facilitate increasing the sensitivity of pressure measurements.
Gas turbine engines typically include a compressor section, a combustor section, and at least one turbine section. The compressor compresses air, which is mixed with fuel and channeled to the combustor. The mixture is then ignited generating hot combustion gases. The combustion gases are channeled to the turbine which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to power a load, such as an electrical generator, or to propel an aircraft in flight.
Gas turbine engines operate in many different operating conditions, and combustor performance facilitates engine operation over a wide range of engine operating conditions. Generally, stable combustion facilitates reducing engine blowout without sacrificing engine rated thrust or power levels. Moreover, in order to meet environmental regulations and to facilitate reducing NOx emissions, at least some known gas turbines are operated with dry low nitrous oxide (DLN) techniques wherein combustion stability also facilitates controlling nitrous oxide (NOx) and carbon monoxide (CO) emissions.
At least some known DLN combustion systems utilize premixed fuel and air, and operate with lean fuel/air ratios (FAR) to facilitate reducing NOx emissions. A lean fuel/air ratio occurs when the ratio of fuel to air is below the stoichiometric value required for the fuel supplied during the present operating condition. However, one consequence of operating with the lean, premixed operation is that the combustion system may operate near a predefined lean blow out (LBO) boundary. Lean blow out or weak extinction is defined as the operating condition during which the mixture of fuel and air is no longer combustible. Moreover within premixed multi-nozzle systems, in particular for those that are piloted by a diffusion or near a stoichiometric flame, weak extinction can be defined as the point at which there is a significant drop in the combustion efficiency and/or complete extinction of the flame.
When operating with lean FAR mixtures, gas turbines may be subject to induced combustion instabilities that are measurable as pressure oscillations. Such oscillations, may be referred to as humming, and may be a precursor to LBO and overtime, may increase to a level known as howling which may cause damage to machinery. In addition, such pressure oscillations may be detectable with a pressure transducer, however the small movement (peak to peak amplitude) of the pressure transducer diaphragm, induced by the humming, in combination with any environmental noise that may be present near the combustor, makes it difficult to detect humming.